The present invention relates to methods and apparatus which can be used to maintain the temperature of a device such as a semiconductor integrated circuit device during a testing operation. In particular, the invention relates to a technique in which the device can be cooled and/or heated during testing in order to control the temperature of the device despite the varying levels of heat generated by the operation of the device.
Control of DUT (xe2x80x9cdevice under testxe2x80x9d) temperature in testing operations has been practiced for some time. During burn-in, for example, the DUT is placed in an environment with an elevated temperature, typically an oven, and signals applied for a prolonged period in order to promote failure of the device which would otherwise occur only after extensive use. The bum-in process uses the elevated temperature to accelerate what would otherwise be a very long-term process within the DUT. In an oven-type bum-in operation, a large number of devices are loaded on burn-in boards which are placed in the oven and tested together.
In other tests, it has been proposed to control the temperature of the DUT so as to simulate possible ambient temperatures during normal use. In such cases, a bulk temperature control is not appropriate because the temperature of each individual DUT can vary significantly over short periods of time. Also, it is not possible to change the temperature of the DUT reliably or quickly during the test process. A number of proposals have been made for more accurate control of DUT temperature during testing. In high-performance ICs, a significant effect of temperature variation during testing is to affect the assessed maximum speed of the device in normal use, known as xe2x80x9cspeed binningxe2x80x9d. Inaccuracy in this assessment can lead to failure of a device in normal use.
U.S. Pat. No. 5,297,621 discloses a liquid bath in which devices are immersed during testing. The liquid in the bath is inert and comprises a mixture of two liquids having boiling points above and below the desired temperature. By varying the mixture of the two liquids, the liquid in the bath is arranged to have a boiling point which is at the desired operating temperature of the DUT (xe2x80x9cset point temperaturexe2x80x9d). Heat generated by the DUT is dissipated by convection within the bath and by nucleate boiling of the liquid on the DUT. Heat transfer from the DUT to the liquid is facilitated by placing a heat sink in contact with the DUT.
U.S. Pat. No. 4,734,872 discloses a system in which a stream of temperature-controlled air is directed at the DUT. The air is drawn into a chiller where its dew point is lowered. The chilled air is then passed to a heater which raises the temperature of the air to a desired level to control the temperature of the DUT. Measurement of the DUT temperature and air stream temperature are used to control the heater and hence the temperature of the air impinging on the DUT.
U.S. Pat. No. 4,784,213 and U.S. Pat. No. 5,205,132 disclose a variant on the system disclosed in the ""872 patent described above. In both of these cases, the air stream is split into a cooled stream and a heated stream. The two streams are then mixed in appropriate proportions to produce a single stream directed at the DUT and having the desired temperature.
U.S. Pat. No. 5,309,090 discloses the heating of a DUT by applying a signal/power to certain structures in the DUT so as to generate heat by their operation and so heat the DUT evenly. This method does not allow the DUT to be cooled in the same way.
Recent developments in high-performance microprocessor design have lead to an increase in power consumption and dissipation from about 10 Watts to 60-70 Watts. Furthermore, the increase in component density within a microprocessor chip and the adoption of modem chip packaging structures has lead to devices which have extremely low thermal inertia, i.e. devices which will heat up and cool down very quickly. CMOS technology used in such chips is characterized by having a power consumption and dissipation which varies depending on the activity of the device. When in normal use, the chips will have cooling devices such as fans mounted nearby or even on the chip package so as to dissipate the heat generated by the operation of these devices. However, during functional testing, these cooling devices are not present and the power dissipated during very high speed functional testing is sufficient to raise the temperature of the device quickly to a level which could permanently damage the device.
The prior art methods of temperature control all rely on a direct measurement of DUT temperature during testing to provide feedback control to the heat exchanger. This approach suffers from a number of problems. It is difficult to make a reliable, consistent temperature measurement at the surface of a device when testing in a high volume manufacturing environment because of the variability in contact resistance encountered. Even with a good temperature measurement, extrapolation of device internal temperature in a high inertia package is problematic. With any feedback system, there is always the problem that the device must change its temperature before the measurement can be made and the thermal response time of the chip can be as low as 30 ms whereas that of the heat exchanger is often in the region 100-200 ms. Therefore, at best, such an arrangement can only smooth temperature variation and can lead to temperature undershoot which is also undesirable.
It is an object of the present invention to provide methods and apparatus which allow temperature control of devices during test, particularly cooling during test.
One aspect of the present invention provides a method for controlling the temperature of a DUT during a testing operation, comprising: a) measuring a parameter related to power consumption by the DUT during testing; and b) using the parameter related to power consumption to operate a temperature control device to compensate for temperature change due to changes in power consumption by the DUT during testing.
Another aspect of the invention provides an apparatus for controlling the temperature of a DUT during testing, comprising: a) means for measuring a parameter related to power consumption by the DUT during testing; b) a temperature control device which operates to control the temperature of the DUT during test; and c) means for controlling operation of the temperature control device according to the measured parameter related to power consumption.
The power supply (device power supply or xe2x80x9cDPSxe2x80x9d) supplies current to the DUT and measuring this current and, optionally, the voltage allows an almost instantaneous indication of the power consumption of the DUT. Since substantially all of the power consumed by the device appears as heat, and the relatively low thermal inertia of such devices, such a measurement also is indicative of heat dissipation by the device and hence its tendency to change temperature. This approach allows the temperature control signal to be generated well in advance of what would be possible using a direct temperature measurement since the parameter being measured exists before the DUT has even changed its temperature. In high performance IC testers, the current consumption (IDD) of the DUT is routinely measured. It is this measurement which can be used to provide the parameter related to power consumption/dissipation of the DUT. The amount of temperature elevation for a given current consumption will depend on a number of factors such as IC package type, form factor, set point temperature etc. In many cases, the effect will be non-linear but can be determined by calibration for a given device type. Where the device has a relatively high thermal inertia, it is desirable to calibrate the performance by including temperature sensors in a test device to allow correlation of IDD with the device internal temperature. The use of IDD measurement is particularly advantageous since this is a parameter which can be accurately and rapidly measured even during high volume testing applications.
The particular form of the temperature control device is a matter of choice. It is necessary that the device can cool the DUT. In order to prevent temperature undershoot, it is also necessary that some form of heating be included to counteract the cooling effect at appropriate times. The choice of temperature control device will affect the manner in which the control signals are used but does not affect the principle of using a power consumption measurement to control operation of the temperature control device. Suitable devices might include the use of mixed heat transfer fluids (hot and cold) to provide the temperature control, or a combination of a cooling fluid and an electric heating element.
While the primary control of temperature is provided by the power consumption measurement, it is also desirable to make actual temperature measurements within the system. The temperature which is critical is that of the DUT and this will depend on the effectiveness of the temperature control device. For example, when the temperature control device includes a heat exchange element which must be placed in contact with the DUT, smoothness of the contacting faces and the contact pressure can affect the effectiveness of heat transfer. Consequently, it may be desirable to include a DUT temperature sensor which can be used to monitor the effectiveness of the temperature control device and provide any offset which may be required to ensure accurate control of the DUT temperature. Measurement of the junction thermal resistance prior to the test in this manner will allow an offset to be determined. Other temperature measurements which might be made are measurements of ambient or set point temperature, temperatures of cooling fluids or temperatures of heat exchangers.
When used in a closed-loop control system, measurement of power consumption replaces direct temperature measurement and so provides faster generation of control signals. However, this approach is still reactive and the slower thermal response time of the temperature control device will still allow some variation in DUT temperature when the DUT thermal response time is faster than that of the thermal control device. Another aspect of the present invention is to use an open loop control system. This is possible because the activity of the DUT is known to some degree in advance from the test patterns applied by the test program. Therefore operation of the temperature control device can be synchronized with the test program. During the development of a test program, measurements of IDD are made and from this the power consumption and hence temperature elevation of the DUT can be determined. Another approach is to measure the temperature of the device during each test segment as it is developed and using this measured temperature profile to control the temperature of the device during testing. By determining the temperature profile during a test segment, a temperature control profile can be derived for that segment to keep the DUT at or near the set point and this can be encapsulated in the test program. In this case, the operation of the temperature control device will be under the control of the tester in the same manner as the functional tests applied to the DUT. Using this approach, changes in DUT temperature can be anticipated and control signal applied to the temperature control device to avoid the delay due to the thermal response time, i.e. in advance of the change in heat change actually experienced by the DUT.
While the active control of the temperature control device described above does not rely on measurement of DUT temperature or power consumption during the test, it is still desirable to make such measurements to account for variation in effectiveness of the temperature control from test to test.